Conductive particles are used in the form that they are interposed between electrode terminals of boards or electronic devices in order to electrically connect a board to another board or to electrically connect a small part such as a semiconductor device to a board. Particularly, the conductive particles are widely used in electronic devices in which a distance between wirings is small, such as a liquid crystal display, a personal computer and a mobile telephone.
An anisotropic conductive material contains a conductive particle and an insulative adhesive resin and used as an electrode-connecting material. Examples of the anisotropic conductive material include an anisotropic conductive film, an anisotropic conductive sheet, anisotropic conductive paste, an anisotropic conductive adhesive, and an anisotropic conductive pressure sensitive adhesive.
Conventionally, the conductive particle prepared by forming a gold layer on its surface has been mainly used. The conductive particle prepared by forming a gold layer on its surface has low connection resistance. Moreover, this conductive particle does not increase in connection resistance even when stored for a long period since there is not a possibility of being oxidized in gold.
However, it is tried to further reduce an amount of a current flowing through an integrated circuit for the purpose of suppressing consumed power in response to energy conservation of recent years. Accordingly, the conductive particle having lower connection resistance than that of a conventional conductive particle has been required.
For this situation, Patent Document 1 discloses a conductive particle prepared by forming a copper layer on the surface of an inorganic or organic particle. Since copper is a metal having smaller electric resistance than that of gold, it was expected to obtain a conductive particle having smaller connection resistance than that of a conductive particle having a gold layer.
However, in fact, the conductive particle provided with the copper layer is sometimes not adequately low in connection resistance. Moreover, there was a problem that copper is apt to be oxidized compared with gold and the connection resistance increases gradually if being stored for a long period.
Patent Document 1: Japanese Kokai Publication Hei-11-61424 (JP-A H11-61424)